1. Field of Invention
The present application relates to a method and an apparatus for setting an active period starting point for a user equipment in a discontinuous reception mode in a mobile communication system, and more specifically, the proposed method and the apparatus may reduce a possibility in which respective active periods of multiple user equipments overlap with one another, so as to improve system capacity and thus achieve efficient utilization of resources.
2. Description of Prior Art
The 3rd Generation Partnership Project (3GPP), which is an important organization in the field of mobile communication, greatly promotes the progress of standardization of the 3rd Generation (3G) mobile communication technologies, and establishes a series of communication system specifications comprising Wide Code Division Multiple Access (WCDMA), High Speed Downlink Packet Access (HSDPA), and High Speed Uplink Packet Access (HSUPA). To deal with the challenges of wideband accesses and to meet increasing demands on new services, 3GPP has initiated the standardization of 3G Long Term Evolution (LTE) at the end of 2004, in order to further enhance spectrum efficiencies, to improve performances of users at edge of a cell, and to reduce system delays, with a final purpose to provide a higher speed access service for users moving at a high speed.
With the communication technologies developing in a direction of mobility and wideband, the problem of power saving in mobile terminals attracts great attentions. Most standardization organizations take power saving into account when stipulating relevant standards. Especially, access networks for future mobile communication systems will all transmit data based on IP technologies. However, data arriving at users are discontinuous, due to the burst property of IP data packets and the property of transmission channels being shared among the users. Therefore, how to save the power consumption of user equipments (UEs) is becoming more important.
In a mobile communication system, information exchange between a UE and a base station is based on power supplied thereto. However, for a mobile terminal such as mobile phone, notebook computer and personal digital assistant (PDA), which is powered by a battery, the stored power is limited. Therefore, it is a key point in designing the mobile communication system to reduce the power consumption so as to prolong the standby and serving time of the UE.
To reduce the power consumption of UEs, a Discontinuous Reception (DRX) mode is adopted in the standards of 3GPP. Specifically, the DRX mode is a mode where the power consumption of a UE is reduced by making the UE monitor a channel and receive downlink traffics at given periods which are negotiated by the UE with a base station, so as to reduce unnecessary time for monitoring the channel. Such DRX mode is also adopted in LTE. As compared with previous standards of 3GPP, an operation process of a UE in the DRX mode is same, which may be characterized by several specific parameters, though the DRX mode has slightly different states, channels and trigger conditions in which it is applied, and the like.
The operation process of a UE in the DRX mode is shown in FIG. 1. As shown in FIG. 1, in the DRX mode, the UE is alternatively in an active period and a sleep period. The interval between two successive active period starting points is referred to as a DRX cycle. In the active period, the UE starts its receiver (Rx) to monitor information on a control channel and receive downlink data. However, in the sleep period, the UE does not need to monitor the control channel, and thus the object of saving power is achieved. In the specification 3GPP TS 36.300 where the LTE is described in general, the DRX of a UE in a Radio Resource Control Connected (RRC_CONNECTED) state is explained and defined as follows:                on-duration: the period during which the UE waits for receiving a Physical Downlink Control Channel (PDCCH) after it wakes up from the DRX, with a unit of Transmission Time Interval (TTI). The UE enters the on-duration when it wakes up from the DRX sleep state. If the UE successfully decodes the PDCCH within such period, then the UE will keep a awaking state (Awake) and starts its inactivity timer; otherwise, the UE will enter the DRX sleep state, if possible under the DRX configuration;        inactivity timer: the period during which the UE waits, from a successful decoding of the PDCCH, for a next successful decoding of the PDCCH, with a unit of TTI. If the UE successfully decodes the PDCCH, the UE will keep the waking state (Awake) and restarts the inactivity timer, until a certain Medium Access Control (MAC) header or control element tells the UE to enter the DRX state again and explicitly indicates the DRX cycle in the MAC payload. Or otherwise, the UE automatically enters the DRX state in accordance with a predetermined DRX cycle when the inactivity timer expires; and        active time: the period during which the UE is in the waking state (Awake), including the on-period and the period during which the UE performs continuous reception before the inactivity timer expires within a DRX cycle. The minimum of the active time is equal to the on-duration period time, and the maximum thereof is not limited.        
By the above described DRX, the UE has not to continuously monitor the channel, but only wakes up intermittently at some certain times, so that the power consumption due to unnecessary monitoring of the channel and decoding of data not pertinent to itself is avoided, resulting in a prolonged standby and serving time of the UE. However, at the same time, the DRX limits the time at which the UE receives the downlink data. For example, in FIG. 1, when the UE is in the sleep period, if data directed to this UE arrives at an eNB (base station), then the eNB must wait for the next active period of the UE to transmit the data to the UE. Thus, this data is delayed. That is to say, the transmission of the downlink data is limited to the active periods of the UE in the DRX state. Each UE has a respective DRX state independently configured by the eNB. When multiple UEs have active periods overlapped, the eNB needs to carry out scheduling and transmitting of data for these multiple UEs within the active periods, while not during other periods. The channels are idle in the periods during which the data cannot be transmitted, resulting in an uneven utilization efficiency of the resources. More severely, the Quality of Service (QoS) may be degraded due to incapability of scheduling a large amount of data within the overlapped active periods.
FIG. 2 is a schematic diagram showing the effect of the DRX of multiple UEs on the data transmission. Here, radio resources are denoted as a set of two-dimensional radio resource blocks, where the horizontal axis represents the time domain with a unit of TTI, and the vertical axis represents the frequency domain with a unit of sub-carrier. Thus, a two-dimensional area consisted of a number of sub-carriers (for example, generally 12 continuous sub-carriers in case of LTE) within one TTI is termed as a Resource Block (RB). The number of RBs within one TTI is denoted by NRB. In FIG. 2, an example of DRX where NRB=3 is schematically shown. The eNB assigns resources based on such set of radio resources, and transmits the data to the respective UEs on the respective RBs. It may be assumed that there are four UEs-UE1, UE2, UE3 and UE4, which are in DRX state and have same DRX cycles and active periods. In such situation, the four UEs (UE1 to UE4) wake up to monitor the channels and receive the data pertinent to them respectively during the same period. At the time t=0, the eNB detects that these UEs are in the active periods and there are data directed to them in memory. Then, the eNB assigns the resources, so that the first, second and third RBs are respectively used for transmit data packets of number 1, 2, and 3, while the data packet of number 4, directed to UE4, is not transmitted due to insufficient radio resources. Since UE4 does not detect the data pertinent there to within this active period, it enters the sleep period. The eNB must wait until UE4 reenters the active period, and then has a chance to schedule the packet of number 4. Thus, the packet of number 4 is delayed by one DRX cycle. Similarly, the packets of number 7 and 8 are also delayed by one DRX cycle. Such delay may degrade the QoS for delay-sensitive services such as voice. At the same time, in spite of insufficient resources during the active period, the RBs are left unused during the sleep periods of the UEs, resulting in resources wasted and thus system capacity reduced. Therefore, it is necessary to change the state of uneven utilization of resource due to DRX, so as to more efficiently utilize the radio resources while saving the power.
As described above, the DRX of a UE depends on the settings of the following parameters: on-duration, inactivity timer, active time, DRX cycle and active period starting point. Among them, the active period starting point determines the timing at which the UE monitors the channels. The problem of uneven utilization of the radio resources may be effectively solved by setting the active period starting point. In prior art, usually the active period starting point of a UE is determined by a unique identification number for the UE and the length of the DRX cycle. For example, the active period starting point is set to be a value of the unique identification number for the UE modulus the length of the DRX cycle. However, as mentioned in Method and Apparatus for Discontinuously Receiving Packet in a Mobile Communication System Appl (Date: 2007 Apr. 11, SAMSUNG ELECTRONICS CO., LTD, [Reference 1]), when setting the active period starting point in such a manner, the time when the data arrives at the eNB has not been taken into account. Since the data may arrive at the memory of the eNB at any time but transmission of them must be postponed until the next active period of the UE, the time of storing the data is prolonged. Especially, for services such as Voice over IP (VoIP), which are delay-sensitive and generate data at a regular interval, the conventional measures for setting the DRX active period starting point increases the delay of packet transmission, and thus degrades the QoS. Therefore, in Reference 1, it is proposed that the active period starting point is set to be the time instant when the UE starts to receive the first packet from the base station, or the time instant when the UE receives the first downlink control channel from the base station if the base station transmits packets using the control element on the downlink control channel. By adjusting the active period starting point in such a manner, it is possible to reduce the time when the data remains in the memory after arriving at the memory, so as to reduce the transmission delay. In this method, only in the case where the UE continuously monitors the channels and the data is ready to be transmitted as soon as it arrives at the memory, the instant when the UE receives the first data is the instant when the data is generated, and thus the object of reducing transmission delay can be achieved. However, if the UE is in the DRX, the first packet only can be received in the active period. Thus, the storage time cannot be reduced. Further, this method cannot solve the problem of uneven utilization of the radio resources. Moreover, while the Qos is satisfied, it has no great effect on the UE's satisfaction to further reduce the packet delay. For example, when the packet delay of 10 ms is reduced to the delay of 5 ms, it can-not be perceived by the user. However, the problem of uneven utilization of the radio resources will reduce the system capacity and the QoS. It is more important to solve the latter, that is, to more evenly and efficiently utilize the radio resources so as to increase the system capacity while ensuring the QoS.
Therefore, there is a need for a method to effectively solve the problem of uneven utilization of the radio resources due to uneven distribution of the active periods, so as to more efficiently utilize the radio resources while saving the power, and to increase the system capacity while ensuring the QoS.